1. Field of the Invention
This invention relates to high damping polymer compositions, and to shaped articles produced therefrom, in particular jacketing for sonar detection arrays.
2. Introduction to the Invention
Many navies throughout the world employ sonar detection arrays to locate and identify vessels at sea. These arrays, which may be 100 meters or more in length, are towed behind a ship by a towline and are isolated from the towline by a vibration isolation module. A detection arrary typically comprises a series of sensing sections, each of which is for example 15 to 30 meters in length. Each sensing section comprises a plurality of hydrophones (located at precise distances from one another), together with the necessary ancillary electronic components, inside a semi-rigid tubular jacket, typically 2 to 10 centimeters in diameter. The tubing is filled with an acoustically transparent fluid, typically a mixture of aliphatic hydrocarbons (although silicone fluids have been proposed), to improve the acoustic coupling between the hydrophones and the water surrounding the tubing.
The tubing often contains, embedded in the wall and running parallel to the longitudinal axis, reinforcing yarn. The main reason for this is to provide the tubing with a very high modulus in the longitudinal direction; if the tubing is readily extensible, this can upset the critical spacing of the hydrophones and can create turbulence, and hence noise, in the acoustic fluid.
It will be readily apparent that a high sensitivity of detection is desirable and that since this sensitivity is partly determined by the signal-to-noise ratio reaching the hydrophones, it is advantageous to reduce the acoustic noise generated within or by the array. As the array is pulled through the water, noise is generated from three separate mechanical excitations. The first is towline excitation originating either at the vessel or as towline strumming due to vortex shedding. The second is a tail end effect; array tension is low and the array may be susceptible to `snaking` instability. The third, and most important, noise source is the turbulent boundary layer (TBL) which develops as the array is drawn through water. The way in which the TBL creates noise is by generating bulge waves (either resonant or non-resonant). The entire success of towed arrays depends on the isolation of the hydrophones from these surface pressures (whose spectral level increases as the fourth power of speed).
It is desirable, then, to use a tubing which provides for maximum transmission of the acoustic signals that the sensors are attempting to detect but which minimizes the noise created by the array. One way of significantly reducing noise is to use a material exhibiting high mechanical damping. It is also important that the material should not be too flexible, as this will more readily permit the generation of bulge waves. Secondary considerations include the density of the material, abrasion resistance, resistance to the fill fluids and water, low temperature flexibility, creep resistance, the ability to take a good surface finish, and (for economic reasons) the ability to manufacture in continuous lengths.
Materials used in the past for jacketing sonar detection arrays include plasticised polyvinyl chloride (PVC) and various rubbers, eg. butyl rubber and nitrile rubber. However, the rubbers suffer from disadvantages such as poor processing characteristics (especially for long lengths), poor abrasion resistance, insufficient stiffness, excessive weight and insufficient solvent resistance. Plasticised PVC can be easily processed, but is too stiff at low temperatures and the plasticiser is leached out by the acoustic fluids.